1. Field of the Invention
The present invention relates to aircraft design, and more particularly to a method and system for utilizing structural feedback, for operating control surfaces on the aircraft to produce stabilizing forces, enabling the aircraft to be designed primarily for strength but not stiffness.
2. Description of the Prior Art
It has been proposed to significantly increase the payload of aircraft by placing all of the payload in the wing of the aircraft so that a close match will be obtained between aerodynamic loading and mass loading. Specific aircraft designs, and the distributed load concept on which such designs are based, are quite thoroughly discussed in an article entitled, Distributed Load Aircraft Concepts, by Philip C. Whitener, appearing in the Journal of Aircraft, Volume 16, No. 2 Feb. 1979, Pages 72-77. This article and the articles and reports mentioned in it are hereby incorporated by reference into this disclosure.
An aircraft built large enough to carry the large payloads described in the aforementioned article would be unacceptably heavy if built stiff enough to avoid aerodynamic stability problems. Also, optimum wing size depends on what unit weight (lbs/sq ft. of area) can be achieved. Larger wing sizes have certain synergistic advantages, listed below, and become optimum with lower unit weights. However, making the wings of a more conventional size airplane larger, with the same or less weight, might present aerodynamic instability problems (including flutter, control reversal, etc., i.e. stiffness problems), along with a need for load distribution control (i.e. strength problems).
A principal object of the present invention is to provide a manner of solving stability problems that would otherwise be associated with the large, limber aircraft of the type mentioned in the foregoing article, or with the more conventional sized aircraft having wings of lower unit weight.
The synerginic benefits arising from lower unit weight of aircraft wings (larger optimum wing size) include:
(1) Higher L/D (b.sup.2 A.sub.wet); PA1 (2) Lower noise; PA1 (3) Simpler flaps; PA1 (4) Shorter field length; PA1 (5) More growth potential; PA1 (6) Better cruise altitude and PA1 (7) Safer low speed performance.
Known systems in which some form of structural feedback is used for positioning control surfaces to produce aerodynamic forces which counter other forces acting on the aircraft structure are disclosed by U.S. Pat. No. 3,279,725 granted Oct. 18, 1966, to Gustav M. Andrew and John M. Johnson, Jr.; by U.S. Pat. No. 3,347,498, granted Oct. 17, 1967 to Eric Priestley and Anthony E. Hodson; by U.S. Pat. No. 3,412,961, granted Nov. 26, 1968 to James C. Howard and by U.S. Pat. No. 3,734,432, granted May 22, 1973, to George M. Low.
Each of the systems disclosed by the above patents uses accelerometers for measuring loads. The system and method of the present invention is specifically addressed to the problem of stabilizing large, structurally limber wings. Accelerometer data is not unique to any specific structure or load direction in a structure and for this reason accelerometer systems are not well suited for measuring the tendency of a wing to bend in response to center line moments.